Eddy current array configuration with reduced length and thickness

ABSTRACT

The invention herein disclosed provides a 2D coil and a method of using the 2D wound EC sensor for reproducing the Eddy Current Testing (ECT) response of a prior art 3D orthogonal sensor. The resulting thin-film eddy current array of coils configured to be placed parallel and against the test surface, and a corresponding eddy current circuitry operable to excite and receive eddy current from the array of coils. The array of coils forming at least a first inspection channel and a second inspection channel. The eddy current circuitry is configured and operable in a way that the one of the first pair of driver coils is usable as one of the second pair of driver coils; and one of the first pair of receiver coils is useable as one the second pair of receiver coils.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit and priority and is a divisionalcontinuation application of U.S. patent application Ser. No. 12/832,620filed Jul. 08, 2010 entitled A 2D COIL AND A METHOD OF OBTAINING ECRESPONSE OF 3D COILS USING THE 2D COIL CONFIGURATION, the entiredisclosure of which is incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to non-destructive testing and inspectionsystems (NDT/NDI), particularly to a method of emulating eddy current(EC) fields generated by a 3D coil using a 2D coil configuration whichcan be fabricated on a printed circuit board (PCB).

BACKGROUND OF THE INVENTION

EC inspection is commonly used in NDT/NDI applications to detect flawsin surfaces of manufactured components fabricated out of conductivematerials, such as steel bars, tubes and pipes. EC is often used toinspect components for automotive, aeronautic and energy industries.Over the years, EC sensors have been designed with differentconfigurations and patterns to suit for different applications.

Various EC systems have heretofore been provided for the detection ofcracks and or other flaws in a part under test. In general, such systemsinclude field producing means such as a coil connected to an AC sourceto generate EC's in a part and a sensing means to sense the fieldproduced by the EC's. The sensing means may be a separate coil, a Hallprobe, or any other field responsive device, or the coil of thefield-producing means may also be used to sense the EC-induced field, bymeasuring the effective impedance thereof

In such prior systems, difficulties are encountered due to the changesin conductivity and permeability of the part under test and also due tothe variations in spacing between the test coil or probe and the surfaceof the part, and variation in surface conditions. It has been possibleto reduce the effect of variations in spacing by certain arrangementssuch as by the use of impedance networks and by adjustment of operatingfrequency. Such arrangements, however, have not overcome the sensitivityto conductivity and permeability changes. To reduce the effect ofconductivity and permeability changes, differentially connected coilshave been used. However, such arrangements have been insensitive todefects common to the differentially connected coils.

Background art has evolved over the years with the general object ofovercoming the disadvantages described above for EC testing systems andproviding systems which are very sensitive to defects while beinginsensitive to variations in other physical characteristics of a partunder test and variations in the physical relation of a test probe tothe part. U.S. Pat. No. 3,495,166 is incorporated by reference as theexample for background art described below.

In accordance with an important feature of the background art, an ECsystem is provided which includes field-sensing means for sensing fieldsproduced by EC's in two regions having substantially the same spatialrelation to a surface of the part and having a substantial angletherebetween with detector means being provided for detectingdifferences between the fields produced in the two regions. It should benoted that the sensing regions of the field-sensing means are orthogonalto the emitted magnetic field regions of field-producing means.Accordingly, in the absence of a defect that will disrupt the directionof the EC flow imparted by the field-producing means, the magnetic fieldresulting from the EC flow will also be orthogonal to the field-sensingmeans and will consequently not be sensed. With this arrangement, a highdegree of sensitivity is obtained with respect to flaws having differentorientations with respect to the sensing regions, while beinginsensitive to changes in a) conductivity, b) permeability, c) irregularsurface finishes and d) to changes in the spacing of the part. Thisinsensitivity stems from the fact that properties a, b, c and d affectpredominantly the magnitude of the EC flow and resulting magnetic field,but not the direction.

It is found that almost all defects which are of interest in the testingof a part have a dimension which is greater in one direction than inanother and with a substantial angle being provided between the sensingregions, a high degree of sensitivity to significant types of defects isobtained. At the same time, the sensing regions can be quite closetogether so as to obtain extremely low sensitivity to variations inspacing or surface conditions, while also obtaining very low sensitivityto changes in conductivity and permeability.

According to another important feature of the background art, thesensing regions are crossed to intersect at mid-points thereof so thatthe area of the part which is inspected is minimized and so that thesensing regions always have the same physical relationship to the partbeing inspected.

According to a specific feature of the background art, the angle betweenthe sensing regions is approximately 90 degrees, to obtain maximumsensitivity to defects.

According to another specific feature of the background art, the sensingregions are relatively long and narrow with transverse dimensions equalto a small fraction of the long dimension thereof, to obtain highresolution and to facilitate detection and location of narrow crackswithin a part.

In accordance with a further feature of the background art, a pair ofcoils are used which are located in planes generally transverse to thesurface of the part.

In certain of the arrangements according to the background art, the pairof coils are used as part of the field-producing means by connectionthereof to an AC source. The same coils may be used as part of thesensing means, or may be used only in the sensing means with anothercoil or coils being used in the field-producing means. In onearrangement, the field-producing means comprise a coil having an axisgenerally parallel to a line at the intersection of the planes of a pairof coils used in the sensing means.

In accordance with an important feature of the background art, the coilshave matched inductances and resistances, to obtain an accurate balanceand to minimize sensitivity to conductivity and permeability variationsand sensitivity to changes in the spacing between coils and the testpart.

In one arrangement in which the same pair of coils are both used infield-producing and field-sensing means, a bridge circuit is providedhaving two branches each having two legs with the two branches beingconnected to an AC voltage source. The pair of coils forms two legs ofthe bridge circuit while impedance means form the other two legs of thebridge circuit and detector means are provided connected between thejunction of the legs of one of the branches and the junction of the legsof the other of the branches. This arrangement further facilitates theattainment of an accurate balance and minimizes sensitivity toconductivity and permeability changes and changes to spacing.

With only one pair of coils, it is possible to miss defects locatedexactly along an angle intersecting the angle between the coils.Although this deficiency is not usually serious, it can be obviated bythe provision of a second pair of coils in planes generally transverseto each other and at angles to the planes of the first pair of coils.

The 3D orthogonal sensor topology described above provides manybenefits; however, a few drawbacks have been known to bring limitationsto its usage. One such drawback is that, with coils wound onto a cube orcross-shaped core, the sensor is inevitably bulky, which limits thespace it can access during inspections. Another drawback is that thefabrication of this sensor largely depends on having the coils manuallywound onto the cubes or the cross-shaped cores. The fabrication is laborintensive and costly.

With the advances of printed circuit board (PCB) technologies over thelast decades, it is now possible to manufacture some EC sensors withcertain coil configurations on a thin, sometimes flexible, support.Significant benefit with the use of PCB technologies to manufacture ECarray probes include reduced manufacturing cost, increased sensorflexibility and increased reproducibility. An example of such a probe isdescribed in U.S. Pat. No. 5,389,876.

A drawback of currently available EC sensors or probes made from printedcircuit boards is that they are limited to simply mapping thetwo-dimensional (2D) shape of the prior art coils that are wound on aplane that is approximately parallel to the inspected surface. This isbecause the printed circuit board is essentially a 2D structure.However, challenges remain in PCB manufacturing for some coilconfigurations such as used in the orthogonal sensors with a 3Dstructure.

The use of solid state magnetic field sensors such as anisotropicmagnetoresistance (AMR) and giant magnetoresistance (GMR), combined withthe printed circuit board technologies, made it possible to obtainprobes with EC responses similar to the conventional orthogonal sensor.An example of this is shown in a patent publication US2005-0007108. Inthis publication, a flat winding coil generates ECs in the componentunder test while a GMR field sensor array picks up the orthogonalmagnetic field generated when a defect disturbs the ECs. While thistechnology benefits some applications, it is unable to provide a fullyflexible probe because the AMR and GMR sensors are discrete componentson the PCB. There are also many limitations intrinsic to AMR and GMRsensors such as the risk of saturation and the need for magneticbiasing, both of which presents undesirable concerns in an industrialenvironment.

Accordingly, it is desirable to provide a method for emulating the ECeffect of a 3D EC sensor structure using a 2D winding configuration,which is suitable to be fabricated using the current printed circuitboard technologies.

It would also be desirable to provide a means for building an EC arrayprobe including sensors that behave in the manner described for the 3Dorthogonal background art sensors with the printed circuit boardtechnology.

SUMMARY OF THE DISCLOSURE

As used herein, the term “3D EC sensor” is an EC sensor comprised of amagnetic field-producing and field-sensing means employed torespectively induce an EC flow in a test object surface and/or sense theresponse field thereof. The magnetic field-producing and field-sensingmeans of the 3D EC sensor are coils, orthogonally disposed or otherwiseprotruding with respect to the test object surface. The EC flow patternproduced on a planar test object surface by the field-producing means ofthe 3D EC sensor is comprised of adjacent regions with their respectiveEC flows in opposite directions. Furthermore, the term “2D EC sensor”should be construed to mean a magnetic field-producing and/orfield-sensing means with similar properties as described above for the3D EC sensor except that the field-producing and field-sensing means aredisposed in a coplaner manner or in close proximity parallel planes suchthat it can be achieved by the use of conventional printed circuit board(PCB) technology.

It is an object of this invention to provide a method for emulating theEC response of a 3D EC sensor structure using a 2D coil or windingconfiguration which is suitable for being fabricated using technologiesinvolving printed circuit board. The 3D EC sensor structure involves atleast a portion of the EC coils that are un-parallel to the surfaceunder inspection.

It is a further object of the present invention to provide a 2D ECsensor to emulate defect patterns nearly identical to its 3D orthogonalcounterpart when used to inspect the target surface.

The method herein disclosed makes it possible to use 2D wound EC sensorsto obtain defect signatures in the impedance plane very similar to, ifnot identical with, those obtained from their conventional 3Dcounterparts.

The significant benefit of the herein disclosed 2D sensor includeslargely reducing the cost associated with manufacturing when using the2D wound EC sensor to replace its 3D counterpart.

Other advantages inherently provided by the herein disclosed 2D wound ECsensors include being able to fabricate using PCB technologies,manufacturing with a fully automated process as compared to manually forits 3D counterpart, and having sensors that are very thin, mechanicallyflexible, if necessary, and easy to be used to access tight spaces forinspection.

Another advantage of the herein disclosed 2D EC sensor is that it allowsthe configuration of adding plurality of layers of such sensors toprovide scanning with higher resolution.

Yet another advantage of the disclosed 2D sensor is that superiorcoupling is obtained between the herein disclosed sensor and the testsurface which provides an increased signal strength.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a (Prior Art) shows a conventional cubic orthogonal 3D coil alongwith a representation of the EC flow generated by the 3D coil onto atest surface.

FIG. 1 b is a perspective view of a basic embodiment illustrating themethod of how to use 2D coil wound on a plane parallel to the testsurface according to the present disclosure, with the generated EC flowimitating its 3D counterpart shown in FIG. 1 a.

FIG. 2 a (Prior Art) shows a conventional cubic orthogonal 3D sensor ina driver-pickup configuration along with a representation of the ECgenerated by this sensor onto the test surface.

FIG. 2 b is a perspective view of the presently disclosed 2D coil in adriver-pickup embodiment wound on a plane parallel to the test surface,with the EC effect imitating its 3D counter part shown in FIG. 2 a.

FIG. 3 illustrates a simplified winding embodiment using the presentlydisclosed 2D coil and reproducing the EC response of an orthogonal 3Ddriver-pickup sensor.

FIG. 4 shows a three channel EC array probe embodiment using amulti-layer 2D coil based on the winding method shown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 a shows an EC flow 3 induced on a conductive test surface 2 by aconventional 3D EC drive coil 1. As can be seen, the driver coil 1 iswound on a 3D cubic core and the winding path includes two planesperpendicular to surface 2. In general, if EC drivers are wound on awinding plane parallel to the test surface, the induced EC flow on thetest surface is parallel to the driver coil. For the case of the 3D ECdrive, drive coil 1 is wound on a 3D structure that contains planes thatare erected from (not parallel to) the test surface, whereas EC flow 3is confined within test surface 2. EC flow 3 can therefore not always beparallel to driver coil 1. Instead, EC flow 3 is driven by theinteraction between surface 2 and the magnetic flow generated by thedriver 1 at surface 2 and forms a pair of whirls on surface 2 as shownin FIG. 1 a.

Referring now to FIG. 1 b, a basic embodiment of the present disclosureis shown to include a pair of whirl-shaped 2D coil wound on a planeparallel to test surface 2 with the generated EC flow 4 conforming tothe EC flow 3 of its 3D counterpart shown in FIG. 1 a. In thisembodiment, EC flow 4 is created using a flat 2D shaped coil 5 built ona plane parallel to the surface 2. It is apparent that EC flow 4 closelyresembles EC flow 3 of FIG. 1 a. Coil 5 is built in a pair winding 5Aand 5B, being oppositely wound, with the look similar to the pair ofwhirl EC flow 3 shown in FIG. 1 a.

Continuing with FIG. 1 b, commercially available simulation tools arefirst used to compute the EC flow generated by conventional coil 1. Theflat 2D winding pattern for coil 5 is then drawn knowing that theinduced EC flow 4 will be substantially parallel to and following coilwinding 5. Taking note that coil 5 includes two half-windings 5A and 5B,which are identical in shape but wound in opposite directions.Alternately, half-windings 5A and 5B may be wound in the same directionand be driven independently with a 180 degree phase difference incurrent to achieve the same effect. This independent drive method may beemployed on a plurality of half-winding pairs such as those describedlater herein with reference to FIGS. 3 and 4.

As can be seen, a higher EC density can be achieved by increasing thewinding density of winding coil 5, particularly as shown in area 32.With the same principle, it is feasible to shape EC flow 4 in order toreproduce, on surface 2, EC flow 3 that was generated by conventionalcoil 1.

It is important to note at this point that the goal of the presentinvention is to use 2D coils to emulate the EC flow distribution onsurface 2 generated by a 3D coil arrangement. However, that is not tosay that the same EC density magnitude is regenerated using the 2Dcoils. In other words, EC flow 4 is a scaled version of EC flow 3. Onlya portion of the magnetic field generated by 3D coil 1 intersects withsurface 2 while a much larger portion of the EC flow generated by 2Dcoil 5 intersects with surface 2 because they are much closer to it.This is a desirable feature of the 2D probe of the present inventionsince it increases the signal strength and thus potentially provides abetter signal to noise ratio.

Still referring to FIG. 1 b, as can be seen, coil 5 can be etched on aflexible or rigid printed circuit board (PCB) with widely used PCBtechnologies. Coil terminals 6 and 7 provide means for connecting thecoils to an EC system acquisition unit (not shown) through separate PCBlayers or by using soldered wires.

Another important aspect of this invention is that a 2D windingconfiguration such as the one used in coil 5 that emulates a driver coilsuch as coil 1 can also be used to emulate a receiver coil with the sameprinciple disclosed above. In other words, a 2D winding pattern such asused in coil 5 can also be used to emulate the readings of a 3D receivercoil whose windings are set on 3D core such as in coil 1.

The geometric characteristics of the 2D flat winding pattern of thepresent invention can be determined by first ascertaining the patternand direction of EC flow 3 resulting from incident magnetic field on adefect free surface 2 that is generated by the 3D driver coil that isthe objective to emulate. After this pattern and direction are known,coil 5 shown in FIG. 1 b is realized by having its windings conform tothe shape and direction of EC flow 3.

It is widely known that a 3D coil configuration shown as in FIG. 1 a canbe used as an “absolute configuration”, which serves both as a drive anda receiver. It is therefore conceivable that, according to the presentlydisclosed invention, one can use winding coil 5 shown in FIG. 1 b bothas a driver and a receiver in an absolute configuration to performinspection for surface 2 and obtain an EC reading emulating the responseof coil 1 that is also used in absolute configuration.

Turning now to FIG. 2 a, a type of a known orthogonal sensor 30 is seeninvolving separate driver coil 1 and receiver coil 29 for inspectingsurface 2. As can be seen, receiver coil 29 is orthogonal to driver coil1. Part of receiver coil 29 is perpendicular to test surface 2 planeonto which EC flow 3 is induced. Using the method herein disclosed andknowing that driver coil 1 and receiver coil 29 are almost identical inshape and number of windings but perpendicular to each other, anequivalent 2D flat winding configuration can be provided by employingtwo perpendicular coils shaped as coil 5 in FIG. 1 b.

As shown in FIG. 2 b, a 2D orthogonal sensor 31 embodies 2D coil 5 asdriver and 2D coil 8 as the orthogonal receiver. Multilayer PCBtechnology allows sensors such as 31 to be manufactured on a singleflexible or rigid PCB assembly by layering the driver and receiverwindings on separate layers and by using the multilayer assembly toconnect on winding leads 6, 7, 9 and 10. As such, the embodiment shownin FIG. 2 b, a 2D orthogonal driver-receiver pair can be used to replacethe conventional 3D orthogonal driver-receiver EC sensor shown in FIG. 2a.

It is worth noting that orthogonal coil configurations such as 30 and/or31 are sensitive particularly within a small area 11 at the center onthe probes. The actual width of this area is preferably defined as beingno more than half of the diagonal of conventional coil 30. For suchconfigurations wherein a small portion of the EC flow is responsible forthe majority of the EC response, it is not as important to preciselymatch the EC pattern over the whole surface 2. With such consideration,the 2D orthogonal coil winding can therefore be modified to a simplifiedwinding pattern as shown in FIG. 3.

As shown in FIG. 3, the alternative 2D orthogonal coil winding isemployed in coil 33. Noting that squared shaped coils 33 are used inlieu of ear-shaped coils as used in sensor 31 of FIG. 2 b to simplifythe design and manufacturing process. Geometries other than square maybe used. Being only a rather rough approximation of the winding patternof sensor 31, sensor 33 produces an EC response on the test surfacesubstantially close to that produced by sensor 31. This is mainly due tothe substantial equivalence in sensitive area 11 in both sensors 31 and33.

Continuing with FIG. 3, it should be noted that sensor 33 includes fourindividual coils. In order to obtain a response equivalent to 3Dorthogonal sensor 30 (in FIG. 2 a), leads 13, 16, 20 and 22 must beconnected to ground. Leads 19 and 17 are connected to the same driversignal (same amplitude and phase). Leads 21 and 23 are connected to adifferential input to provide a single receiver signal. The benefit ofthis alternate method for connecting these windings will become apparentwhen considering the following disclosed array version of the flatorthogonal sensor.

Reference now is made to FIG. 4. In FIG. 4, array probe 34 comprises acoil configuration for building a compact EC array probe using multiple2D coils as disclosed above in association with FIGS. 2 b and 3 toreproduce the EC response generated by a 3D EC array probe. Morespecifically, probe 34 includes four 2D driver coils (12, 13, 25 and 26)and four 2D receiver coils (14, 15, 27 and 28). Multiplexer unit 24 canactivate each driver winding and the corresponding pairs of receiverwindings (14-15; 27-15; 27-28) sequentially. As seen in FIG. 4, thefirst inspection channel is generated by activating drivers 12 and 13along with receivers 14-15 and provides a sensitive spot 35. The secondinspection channel is generated by activating driver 13 and 25 alongwith receivers 27-15 and provides a sensitive spot 36. The thirdinspection channel is generated by activating driver 25 and 26 alongwith receivers 27-28 and provides a sensitive spot 37.

The above exemplary 2D EC array probe 34 shown in FIG. 4 is a threechannel array probe. It should be noted that that any number of channelscan be used to build such an array probe depending on the application.

Since multiplexer unit 24 and an acquisition unit (not shown) capable ofsupporting array probes such as 34 are commercially available, theirdetails are therefore not elaborated here. It must also be understoodthat the two-layer PCB structure disclosed herein is exemplary. Morelayers of PCB design can also be used. For example, each coil could makeuse of several layers of PCB to increase the probe inductance whichallows the use of lower test frequencies. Another example would be tostack several staggered probes such as 34 over the multi-layer structureto provide a better coverage (higher resolution) of the inspectedsurface.

Yet another example would be to use different PCB layers for detectingdefects of different orientations by using an adapted coil pattern.

It should be further noted that, since it is known that conventional 3Dorthogonal sensors can by connected in transmit-receive or indifferential configurations, it is conceivable that the presentlydisclosed 2D orthogonal counterpart can likewise be connected in adifferential configuration to provide a sensitivity axis shifted by 45degrees, and that a corresponding array probe configuration can also bemade.

As herein disclosed that equivalent windings, such as 5 in FIGS. 1 b and2 b, or sets of windings such as 12 and 18 in FIG. 3, can be found toemulate any coils included in a given 3D sensor by obtaining equivalentEC patterns on the inspected surface as described in this invention, itis within the scope of the present invention that a wide range of coilconfigurations designed for the 3D sensors may be replicated with its 2Dcounterpart. The key is that the herein disclosed 2D flat sensors havethe capability to provide an equivalent EC response on the surface beinginspected and thus emulate the defect signatures as provided by the 3Dcounterpart in the impedance plane.

It is also important to note that coils 5A and 5B in FIG. 1 b havesubstantially the same pattern. However, if driver/receiver coils to beemulated are of irregular patterns or are situated on the test surfaceasymmetrically, the EC flow resulting from the 3D coils will be ofasymmetrical irregular patterns. The scope of the present disclosurepertains to the pattern of the EC flow generated by the 3D coils whichare further used to determine the pattern of the 2D coils.

Although the present invention has been described in relation toparticular exemplary embodiments thereof, many other variations andmodifications and other uses will become apparent to those skilled inthe art. It is preferred, therefore, that the present invention not belimited by the specific disclosure.

What is claimed is:
 1. An eddy current array probe assembly configuredto perform eddy current inspection of a test object with a test surface,the assembly comprising: a thin-film eddy current array of coilsconfigured to be placed parallel and against the test surface, and acorresponding eddy current circuitry operable to excite and receive eddycurrent from the array of coils, the array of coils forming at least afirst inspection channel and a second inspection channel, wherein thefirst inspection channel further comprising a first pair of driver coilsand receiver coils, wherein centers of the first pair of driver coilsform a first driver axis and centers of the first pair of receiver coilsform a first receiver axis; wherein, in a plan view taken in a directiontowards the test surface, the first driver axis and the first receiveraxis cross and are orthogonal to each other; wherein the secondinspection channel further comprising a second pair of driver coils andreceiver coils, wherein centers of the second pair of driver coils forma second driver axis and centers of the second pair of receiver coilsform a second receiver axis; wherein, in a plan view taken in adirection towards the test object, the second driver axis and the secondreceiver axis cross and are orthogonal to each other; wherein the eddycurrent circuitry is configured and operable in a way that the one ofthe first pair of driver coils is usable as one of the second pair ofdriver coils; and one of the first pair of receiver coils is useable asone the second pair of receiver coils.
 2. The eddy current array probeassembly in claim 1, wherein the first pair of driver coils are a pairof substantially identical, yet reversely wound driver coils and thefirst pair of receiver coils comprising a pair of substantiallyidentical, differentially connected receiver coils; and, wherein thesecond pair of driver coils are a pair of substantially identical, yetreversely wound driver coils and the second pair of receiver coilscomprising a pair of substantially identical, differentially connectedreceiver coils.
 3. The eddy current array probe assembly in claim 1,wherein the eddy current circuitry is configured to excite the firstpair of driver coils to cause an eddy current flow in the test object tobe in a pattern of a pair of substantially identical yet reversely woundswirls; and, the eddy current circuitry is configured to excite thesecond pair of driver coils to cause an eddy current flow in the testobject to be in a pattern of a pair of substantially identical yetreversely wound swirls.
 4. The eddy current array probe assembly inclaim 1, wherein the first pair of driver coils and receiver coils areover-laid on top of each other and the second pair of driver coils andreceiver coils are over-laid on top of each other.
 5. The eddy currentarray probe assembly in claim 1, wherein the array of coils is of a typefabricated using printed circuit board manufacturing technologies. 6.The eddy current array probe assembly in claim 1, wherein each of atleast some of the array coils comprises at least first and secondseparate coil portions, each portion being wound in the same direction,driven independently with a 180 degree phase difference, the two coilportions being disposed on the same plane parallel to the test surface.7. The eddy current array probe assembly in claim 1, wherein the arraycoils are formed by employing one of the array coils multiplied into aplurality of identical 2D coils arranged in a staggered manner in aseries and/or in multiple layers.